Various kinds of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of which have already been put into practical use. The most widely employed precursor is a light-sensitive material having a photoconductive layer comprising photoconductive particles, such as zinc oxide, and a binder resin provided on a conductive support. A highly lipophilic toner image is subsequently formed on the photoconductive layer surface by an ordinary electrophotographic process. The surface of the photoconductive layer having the toner image is then treated with an oil-desensitizing solution, called an etching solution, to selectively render the non-image areas hydrophilic thereby producing an offset printing plate.
In order to obtain satisfactory prints, an offset printing plate precursor or light-sensitive material must faithfully reproduce an original on the surface thereof; the surface of the light-sensitive material should have a high affinity for an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic and, at the same time, should be water resistant. When used as printing plate, the photoconductive layer having a toner image formed thereon should not come off during printing, and should be well receptive to dampening water so that the non-image areas can remain sufficiently hydrophilic to be free from stains, even after a large number of prints have been reproduced from the plate.
These properties are affected by the proportion of zinc oxide to binder resin in the photoconductive layer as already known. Specifically, when the proportion of zinc oxide particles to binder resin in the photoconductive layer is decreased, the oil-desensitivity of the photoconductive layer surface is enhanced and background stains are decreased. However, the internal cohesive force and mechanical strength of the photoconductive layer itself is lowered resulting in the deterioration of the printing durability. On the contrary, when the proportion of a resin binder is increased, the background stains are increased although the printing durability is heightened. Background stains are related to the oil-desensitivity of the photoconductive layer surface. Not only does the ratio of zinc oxide to binder resin in the photoconductive layer influence the oil-desensitivity, but it has become apparent that the oil-desensitivity also depends greatly on the kind of the binder resin employed.
Known resins for use in photoconductive layers include silicone resins as disclosed in JP-B-34-6670 (the term "JP-B" as used herein means an "examined Japanese patent publication"), styrene-butadiene resins as disclosed in JP-B-35-1950, alkyd resins, maleic acid resins and polyamides as disclosed in JP-B-35-11219, vinyl acetate resins as disclosed in JP-B-41-2425, vinyl acetate copolymers as disclosed in JP-B-41-2426, acryl resins as disclosed in JP-B-35-11216, acrylic acid ester copolymers as disclosed, for example, in JP-B-35-11219, JP-B-36-8510, and JP-B-41-13946. However, electrophotographic light-sensitive materials employing these resins have various problems including (1) low chargeability of the photoconductive layer, (2) poor image reproducibility (in particular, dot reproducibility and resolving power), (3) low photosensitivity, (4) insufficient oil-desensitivity of the photoconductive layer surface resulting in generation of background stains on the prints when offset printing is performed even when subjected to an oil-desensitizing treatment for producing an offset master, (5) insufficient film strength of the photoconductive layer, resulting in peeling off of the photoconductive layer during offset printing, and a large number of prints can not be obtained, and (6) the image quality is apt to be influenced by the environmental condition at the time of image reproduction (e.g., high temperature and high humidity condition).
With respect to the offset master, the background stain resulting from insufficiency in oil-desensitization is a particularly serious problem. For the purpose of solving this problem, as binder resins for zinc oxide, various binder resins have been developed for improving the oil-desensitivity. Resins having an effect on improvement in oil-desensitivity of the photoconductive layer include those as follows: JP-B-50 31011 discloses the combination of a resin having a weight average molecular weight of from 1.8.times.10.sup.4 to 1.0.times.10.sup.5 and a glass transition point (Tg) of from 10.degree. C. to 80.degree. C., and which is prepared by copolymerizing a (meth)acrylate monomer and another monomer in the presence of fumaric acid, with a copolymer prepared from a (meth)acrylate monomer and a monomer other than fumaric acid; JP-A-53-54027 (the term "JP-A" as used herein means an "unexamined published Japanese patent application" discloses a terpolymer comprising a (meth)acrylic acid ester unit having a substituent which contains a carboxylic acid group apart from the ester linkage by at least 7 atoms; JP-A-54-20735 and JP-A-57-202544 disclose a tetra- or penta-polymer comprising an acrylic acid unit and a hydroxyethyl (meth)acrylate unit; and JP-A-58-68046 discloses a tercopolymer comprising a (meth)acrylic acid ester unit having an alkyl group containing from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxylic acid group. However, even with the practical use of the above-described resins, which are described to enhance oil-desensitivity, the resulting offset masters are still insufficient in resistance to background stains and printing durability.
On the other hand, resins of the type which contain functional groups capable of producing hydrophilic groups through decomposition have been investigated on an aptitude for the resin binder. For example, the resins containing functional groups capable of producing hydroxy groups by decomposition are disclosed in JP-A-62-195684, JP-A-62-210475 and JP-A-62-210476, those containing functional groups capable of producing carboxy groups through decomposition are disclosed in JP-A-62-212669, JP-A-1-63977 and JP-A-62-286064, and those containing functional groups capable of producing hydroxy groups or carboxy groups through decomposition and having crosslinking structure therebetween which restrains the solubility thereof in water and impart water swellability thereto, whereby the prevention of background stains and the printing durability are furthermore improved as disclosed in JP-A-1-191157, JP-A-1-197765, JP-A-1-191860, JP-A-1-185667, JP-A-1-179052 and JP-A-1-191158.
However, when these resins are practically employed as the binder resin of lithographic printing plate precursor in an amount sufficient to increase the hydrophilic property of the non-image areas and to prevent background stains, the electrophotographic characteristics (particularly, dark charge retention property and photosensitivity) are fluctuated and good duplicated images can not be stably obtained sometimes in a case wherein the environmental conditions at the image formation are changed to high temperature and high humidity or to low temperature and low humidity. As a result, the printing plate precursor provides prints of poor image or having background stains.
Further, when a scanning exposure system using a semiconductor laser beam is applied to digital direct type electrophotographic lithographic printing plate precursor, the exposure time becomes longer and also there is a restriction on the exposure intensity as compared to a conventional overall simultaneous exposure system using a visible light, and hence a higher performance has been required for the electrostatic characteristics, in particular, the dark charge retention property and photosensitivity.
However, when the above-described lithographic printing plate precursors containing known resins are employed in the scanning exposure system described above, the electrophotographic characteristics degrade, and the occurrence of background fog, cutting of fine lines and spread of letters are observed in the duplicated image obtained. As a result, when they are employed as printing plates, the image quality of prints obtained becomes poor, and the effect of preventing background stains owing to the increase in hydrophilic property in the non-image areas due to the binder resin is lost.